Cellulose based electrically insulating material

1. An electrical transformer, comprising:

an oil tank filled with oil; and

a cellulose based electrically insulating composite material disposed in the oil tank, the composite material comprising:

cellulose fibres in an amount of 85 to 99 wt % of the composite material; and

an electrically insulating thermoplastic polymer material in an amount of 1 to 15 wt % of the composite material,

wherein the thermoplastic polymer material is arranged around and between the cellulose fibres, and binds said fibres to each other,

wherein the composite material is at least partly impregnated with the oil, and

wherein the thermoplastic polymer material has a melting point of between 140° C. and 200° C., and

wherein the composite material is configured to shrink less than 0.5% in response to drying the transformer.

2. The electrical transformer of claim 1 , wherein the composite material is in the form of a spacer, barrier, strip or press ring.

3. The electrical transformer of claim 1 , wherein the thermoplastic polymer material has a glass transition temperature of between 50° C. and 120° C.

4. The electrical transformer of claim 1 , wherein the thermoplastic polymer material comprises polylactic acid.

5. The electrical transformer of claim 1 , wherein the composite material has an open foam structure.

6. The electrical transformer of claim 1 , wherein the thermoplastic polymer material is permeable to at least one of air, water, or oil.

7. The electrical transformer of claim 1 , wherein the thermoplastic polymer material includes poly-L-lactide.

8. The electrical transformer of claim 1 , wherein the thermoplastic polymer material includes polyethylene terephthalate.

9. The electrical transformer of claim 1 , wherein the cellulose fibres are present in the composite material in an amount of 90 to 95 wt % of the composite material, and

wherein the thermoplastic polymer material is present in the composite material in an amount of 5 to 10 wt % of the composite material.

10. The electrical transformer of claim 9 , wherein the thermoplastic polymer material includes poly-L-lactide in an amount of 5 to 10 wt % of the composite material.

11. The electrical transformer of claim 1 , wherein the composite material is formed using a method comprising:

providing a pulp comprising the cellulose fibres;

mixing the pulp with the thermoplastic polymer material to form a mixture;

pressing the mixture in a paper press to form a pressboard, wherein the pressing includes:

heating the mixture such that the thermoplastic polymer material melts to lay around and between the cellulose fibres;

drying the mixture to remove moisture from the mixture;

pressing the mixture into the pressboard; and

cooling the pressboard such that the melted thermoplastic polymer material solidifies such that the thermoplastic polymer material is arranged around and between the cellulose fibres to coat said fibres and bind said fibres to each other and to form a matrix of the cellulose fibres and the thermoplastic polymer material; and

impregnating the composite material at least partly with oil.

12. A cellulose based electrically insulating composite material for a transformer, the composite material comprising:

cellulose fibres; and

an electrically insulating thermoplastic polymer material having a melting point of between 140° C. and 200° C.,

wherein the polymer material is arranged around and between the cellulose fibres, and binds said fibres to each other wherein the cellulose fibres are present in the composite material in an amount of 85 to 99 wt % of the composite material, wherein the thermoplastic polymer material is present in the composite material in an amount of 1 to 15 wt % of the composite material,

wherein the composite material is configured to be at least partly impregnated with an insulating substance, and

wherein drying the composite material after being at least partly impregnated with the insulating substance causes the composite material to shrink by less than 0.5%.

13. The composite material of claim 12 , wherein the thermoplastic polymer material includes poly-L-lactide.

14. The composite material of claim 12 , wherein the cellulose fibres are present in the composite material in an amount of 90 to 95 wt % of the composite material, and

wherein the thermoplastic polymer material is present in the composite material in an amount of 5 to 10 wt % of the composite material.

15. The composite material of claim 14 , wherein the thermoplastic polymer material includes poly-L-lactide.

16. The composite material of claim 12 , wherein the composite material is formed using a method comprising:

providing a pulp comprising the cellulose fibres;

mixing the pulp with the polymer material to form a mixture;

pressing the mixture in a paper press to form a pressboard, wherein the pressing includes:

heating the mixture such that the thermoplastic polymer material melts to lay around and between the cellulose fibres;

drying the mixture to remove moisture from the mixture;

pressing the mixture into the pressboard; and

cooling the pressboard such that the melted polymer solidifies such that the polymer material is arranged around and between the cellulose fibres to coat said fibres and bind said fibres to each other and to form a matrix of the cellulose fibres and the thermoplastic polymer material; and

impregnating the composite material at least partly with oil.

17. The composite material of claim 16 , further comprising:

drying the composite material to reduce a water content of the composite material from about 2% to about 0.5%, wherein the drying causes a thickness dimension of the composite material to shrink by less than 0.5%.

18. A method of producing a cellulose based electrically insulating composite material for an oil impregnated electrical transformer, the method comprising:

providing a pulp comprising cellulose fibres;

providing an electrically insulating thermoplastic polymer material having a melting point of between 140° C. and 200° C.;

mixing the pulp with the thermoplastic polymer material to form a mixture;

pressing the mixture in a paper press to form a pressboard, wherein the pressing includes:

heating the mixture such that the thermoplastic polymer material melts to lay around and between the cellulose fibres;

drying the mixture to remove moisture from the mixture;

pressing the mixture into the pressboard; and

cooling the pressboard such that the melted thermoplastic polymer material solidifies such that the thermoplastic polymer material is arranged around and between the cellulose fibres to coat said fibres and bind said fibres to each other and such that the composite material includes the cellulose fibres in an amount of 85 to 99 wt % of the composite material and the thermoplastic polymer material in an amount of 1 to 15 wt, and such that drying the composite material after impregnating the composite material at least partly with oil causes the composite material to shrink by less than 5%; and

impregnating the composite material at least partly with oil.

19. The method of claim 18 , wherein the mixing includes adding and mixing the thermoplastic polymer material with the cellulose fibres in a solvent process and wherein the cellulose fibres are coated with the thermoplastic polymer material, as fibres of the thermoplastic polymer material or as a powder of the thermoplastic polymer material.

20. The method of claim 18 , wherein the mixing comprises adding the thermoplastic polymer material to the pulp comprising the cellulose fibres, before or after grinding of the pulp.